Intense terahertz emitters are one of the most important components of terahertz (THz) time-domain spectroscopy systems. In this presented report, the development of THz emitters over the last two decades is reviewed, and an outlook for future THz emitters is given. The physical principle behind the THz generation process is discussed for two types of emitters: state-of-the-art large-area photoconductive emitters are compared to THz emitters based on the photo-Dember effect. The latter do not require an external bias voltage. This passive character of the photo-Dember emitters has several advantages which are outlined.